1. Field of the invention
The present invention relates to a vehicular radar device installed on, for example, a vehicle and operative to detect the presence or absence of a preceding vehicle or the like and position information on the same by using electromagnetic waves such as laser light.
2. Description of the Related Art
Wide advances have heretofore made in the development of radars to be used in vehicles for purposes such as monitoring of forward obstacles and adaptive cruise control (vehicular radar devices), and a radio wave system and a laser system are known. Each of these systems is constructed to transmit an electromagnetic wave such as a radio wave or laser light toward an object within a predetermined detection area, and finds the distance to the target from a reception delay time which elapses until the radar receives a reflected wave of the electromagnetic wave. There is also another system which detects the direction and the like of a target by transmitting an electromagnetic wave while scanning it.
In such a vehicular radar device, as a matter of course, rain, snow, fog and the like as well as a road surface need to be detected in the state of being discriminated from a vehicle and the like, so that their measured data are eliminated from monitoring targets to prevent rain, snow, fog and the like as well as a road surface from being erroneously detected as a monitoring target (such as an objective vehicle or an obstacle).
In addition, since this kind of vehicular radar device (a laser radar in particular), when it is influenced by rain, snow, fog or the like, suffers a lowering in detection distance performance, rain or the like needs to be detected in the state of being discriminated from a vehicle or the like which is a monitoring target. As shown in FIG. 7A by way of example, when the detection distance performance of a radar device provided in a vehicle C1 is lowered by unfavorable weather such as rain, the radar device will not be able to detect a preceding vehicle C2, until the distance of the preceding vehicle C2 from the vehicle C1 approaches a position which lies at a shorter distance as shown by symbol C3. Accordingly, in this situation, it is desirable to take measures such as the processing of making the transmission power and the reception sensitivity of the radar device higher than those under normal conditions so as to maintain the distance performance at approximately the same level as that under normal conditions, or the processing of outputting a warning informing a driver that the distance performance has lowered.
FIG. 8B shows data obtained by actually measuring a received light waveform in the case where a predetermined target is installed at a short distance (20 m) under unfavorable weather conditions entailing a rainfall of 30 mm per hour (in FIG. 8A, the horizontal and vertical axes represent distance (m) and received light quantity, respectively). FIG. 8C shows data obtained by similarly actually measuring a received light waveform in the case where a predetermined target is installed at a long distance (40 m) under similar unfavorable weather conditions. It can be seen from FIGS. 8B and 8C that under the unfavorable weather conditions, the waveform of the target installed at the short distance exceeds the threshold of the received light quantity and can be detected, whereas the waveform of the target installed at the long distance does not exceed the threshold of the received light quantity and cannot be detected, so that the detection distance performance becomes low under the unfavorable weather conditions. The threshold is preset on the basis of experiments and the like so that erroneous detection due to noise can be eliminated.
To meet this demand, Patent Document 1 (Japanese Patent No. 2,776,253) proposes a laser radar which makes a decision as to reflected light from fog or the like on the basis of a received waveform.
Patent Document 2 (JP-A-2000-65932) proposes a vehicular optical radar device which determines that a detection target is snow or fog, when it is recognized that the distance of the detection target is approximately the same over a predetermined range of scanning angles (or it is recognized that the relative speed of the detection target is zero).
Patent Document 3 (JP-A-9-159765) proposes a vehicular radar device which sets reference data on the basis of measured data determined to contain no reflected light from an object and corrects measured data on the basis of this reference data, thereby extracting only reflected light of the object from measured data including scattered light due to snow or waterdrops.
Patent Document 4 (JP-A-2000-40139) proposes a method of recognizing an object ahead of a vehicle, which prevents a non-monitoring target such as splashes from being detected, as by relatively lowering the output level of and reception sensitivity to a transmitted wave in a region where there is a low possibility that a monitoring target such as a vehicle may exist.
Patent Document 5 (JP-A-2003-42757) proposes a vehicular distance measuring device which determines whether a detection target is a road surface, on the basis of a detected distance and a received waveform (a feature quantity of reflected light).